A typical construction for a heavy truck is based on a steel chassis frame. A cab for the driver is mounted atop the frame. An engine is mounted on the frame in front of the cab. Suspension systems for the wheels attach to the frame. Various components of a drivetrain that couples the engine to driven wheels are supported on the frame, as are components of various other systems such as brake and steering systems. In the case of a highway tractor, a fifth wheel is mounted atop the rear of the frame.
A common construction for a chassis frame comprises right and left frame rails that run lengthwise of the truck. At various locations along their lengths, the rails are bridged by cross members. Assembly of a truck frame at a truck assembly plant is rather labor- and part-intensive because the ends of the cross members are typically fastened to the rails by a number of fasteners. Longer truck frames usually have more cross members, and hence their assembly requires more fasteners and fastening operations, running into the hundreds of parts. The process also uses a significant amount of plant floor space.
Forces and torques that are exerted on the chassis frame when the vehicle is being driven cause some bending and twisting of the frame. Extreme forces and torques cause more severe frame deformation. Over time, repeated frame deformations may degrade the quality of ride and road handling performance.